The present invention is directed to reflective cholesteric liquid crystal displays and, in particular, to the use and treatment of alignment layer materials to improve performance of such displays.
Liquid crystal displays have brought to the world a low-power, flat-screen technology that has enabled many new portable devices. As the demand for more portable devices grows, so does the demand for displays with improved performance. Reflective displays are important not only because they reduce power consumption, but also because they can be clearly seen in sunlight. Displays with bistable memory are attractive for reducing power consumption and increasing battery life compared to displays in which the image needs to be continuously refreshed.
Bistable liquid crystal displays were developed using cholesteric liquid crystalline materials. With a low concentration polymer network, both the focal-conic and planar textures of a cholesteric liquid crystal can be made stable and it is possible to electrically switch between the two states. Reflective cholesteric displays that employ polymer may use cells with rubbed substrates to achieve a homogeneous alignment of the liquid crystal at the surface of the cell substrate. Homogeneous alignment tends to favor the planar texture. The polymer network prevents the focal-conic texture from transforming to the planar texture.
It was discovered that a bistable cholesteric display could be made without a polymer network if inhomogeneous surfaces for the cell substrate were employed. In this case, cell substrates were typically unrubbed. U.S. Pat. No. 5,453,863 describes polymer-free cholesteric display cells which are said to be bistable and can be electrically switched between the two optical textures. The U.S. Pat. No. 5,453,863 discloses that alignment layers of a cell may be rubbed or unrubbed. However, the U.S. Pat. No. 5,453,863 patent does not address the relationship between display performance and variations in the degree of rubbing and choice of alignment layer materials. More importantly, that patent does not disclose how to optimize brightness by increasing the reflectivity of the planar texture or how to optimize contrast by lowering reflectance of the focal conic state. A display with a focal conic state that has high reflectance will have low contrast and is of little commercial value. The U.S. Pat. No. 5,453,863 patent also does not describe how to control the degree of circular polarization of the reflected or transmitted light important in stacked cell configurations.
Cholesteric liquid crystal displays have been popular for portable signs as well as hand held devices where low-power consumption and hence long battery life is important. Being reflective, these displays are readable in bright sunlight as well as room light. A drawback with current cholesteric liquid crystal displays, however, is that their brightness and contrast are not optimized. The brightness is reduced by the defects introduced in the planar texture by the polymer network or by an inhomogeneous surface alignment such as the unrubbed surface in polymer-free displays. Defects also tend to destroy the polarization state of the reflections from the planar texture. Likewise, contrast has been limited by the light scattering nature of the focal-conic texture. Defects in the focal-conic texture affect the light scattering properties of that texture. It is desirable that the focal-conic texture be as transparent as possible in order to show the black or colored back-layer as clearly as possible.
Cholesteric liquid crystal displays may achieve bistability of the focal conic and planar textures through the use of an inhomogeneous aligning surface rather than with polymer present in the bulk of the liquid crystal. An inhomogeneous aligning surface that is typically used in cholesteric liquid crystal displays is in the form of an unrubbed polyimide layer. There are various types of polyimide alignment layer materials characterized by different pretilt angles, or angles by which the liquid crystal director extends from the surface of the substrate. The lack of rubbing results in an azimuthally random distribution of the nematic director at the surface (inhomogeneous alignment). In this case, both the planar and focal-conic textures are stable under zero field conditions. This inhomogeneous alignment layer introduces defects in the uniform planar and focal conic textures, giving them stability, i.e., the ability to maintain set optical textures in the absence of an electric field.
It is conventionally believed that a very high brightness xe2x80x9cperfect planarxe2x80x9d texture may be induced by homogeneous alignment of the liquid crystal director. However, such an alignment condition alone does not stabilize the focal-conic texture and provide it with desirable low reflectance. It is well known that light reflected from a perfect single domain planar texture is completely circularly polarized. Defects resulting from an inhomogeneous alignment surface or a polymer network substantially reduce the degree of circular polarization of the reflected and transmitted light.
The present invention is directed to chiral nematic liquid crystal displays which include a xe2x80x9chomogeneousxe2x80x9d alignment surface on one or both of the substrates (i.e., sides) of a cell. This surface tends to align the liquid crystal director adjacent thereto and provide the display with increased brightness, low focal conic reflectance and/or reflected light that has an increased degree of circular polarization. Aspects of the present invention include a display with one side treated; a display with both sides treated; orientations of a display with the untreated side located nearest to and farthest from a viewer; and a stacked display having a cell with at least one side treated, such as a stacked display in which a second (e.g., lower) cell has both sides treated and a first (e.g., upper) cell has only the side nearest the second cell treated. These different aspects of the invention may be achieved through the use of various alignment techniques such as rubbed polyimide, UV alignment, selection of alignment material such as low or high pretilt, and combinations of the foregoing.
Increased performance of the inventive liquid crystal display is intended to refer to increase compared to a control liquid crystal display device that is identical but has inhomogeneous alignment surfaces such as unrubbed polyimide. With regard to a stacked display, a corresponding cell of the control device should be referred to for purposes of comparison. For example, if a bottom cell of an inventive stacked display has increased brightness, the brightness of a bottom cell of the stacked control display should be considered. Increases of properties are calculated relative to an increase in percentage or degree of the magnitude of the property that is achieved by the control display. For example, an inventive 30% increase in a property, such as brightness, means a brightness that is 1.3 times the brightness of the control display. If not otherwise indicated, it is should be understood that increases or decreases in a property are with regard to the property of the control display.
One embodiment of the present invention is directed to a liquid crystal display having at least one cell with at least one side treated so as to enhance brightness, comprising chiral nematic liquid crystal material having positive dielectric anisotropy. In all embodiments of the present invention, the liquid crystal material is preferably substantially free from polymer. Cell wall structure contains the liquid crystal material. At least one homogeneous alignment surface is effective to substantially homogeneously align the liquid crystal director adjacent thereto. At least one of the cell wall structure and each homogeneous alignment surface cooperates with the liquid crystal material so as to form focal conic and planar textures that are stable in the absence of a field. This homogeneous alignment surface is effective to increase brightness by at least 5% at a wavelength of peak reflection of the planar texture over the reflectance of the planar texture in the control display. More specifically, brightness may be increased by at least 15% and, more preferably, by at least 30%. A device is used for applying an electric field to transform the liquid crystal material to at least one of the focal conic and planar textures.
Another embodiment of the present invention is directed to a liquid crystal display device having a focal conic state of low reflectance, comprising the chiral nematic liquid crystal material, the cell wall structure and the device for applying the electric field described above. At least one homogeneous alignment surface is effective to align the liquid crystal director adjacent thereto. At least one of the cell wall structure and each homogeneous alignment surface cooperates with the liquid crystal material so as to form focal conic and planar textures that are stable in the absence of a field. This homogeneous alignment surface is effective to prevent reflectance by the focal conic texture from exceeding 10% of electromagnetic radiation incident on the display at a wavelength of peak reflection of the planar texture. More specifically, in this embodiment each homogeneous alignment surface may cooperate with the material so as to be effective in increasing brightness by at least 5% at a wavelength of peak reflection of the planar texture. More specifically, brightness may be increased by at least 15% and, more preferably, by at least 30%. In all embodiments of the present invention the inventive liquid crystal display device is characterized by a threshold voltage for multiplexing.
In both of the enhanced brightness and low focal conic reflectance embodiments, the cell wall structure may comprise opposing substrates. A homogeneous alignment surface in the form of a rubbed alignment layer may be disposed adjacent one of the substrates, an inhomogeneous alignment surface being located on the opposing substrate (i.e., a cell treated on one side). In another aspect, homogeneous alignment surfaces in the form of rubbed alignment layer materials are disposed on both substrates (i.e., a cell treated on both sides). The homogeneous alignment surface may be in the form of a rubbed alignment layer material such as polyimide in all aspects and embodiments of the invention.
The liquid crystal material may be selected from the group consisting of various chiral nematic liquid crystal materials each having a pitch length effective to reflect a selected wavelength of electromagnetic radiation, such as at least one of visible and infrared radiation. The device for applying an electric field is effective to provide the liquid crystal material with stable gray scale states. In all embodiments of the invention in which only one substrate of a cell is treated, the untreated substrate may be either upstream or downstream of the homogeneous alignment surface relative to a direction of light incident to the display.
Another embodiment of the present invention relates to a liquid crystal display in which reflected light is to a significant degree circularly polarized, comprising the chiral nematic liquid crystal material, cell wall structure and device for applying the electric field discussed above. At least one homogeneous alignment surface is effective to align the liquid crystal director adjacent thereto. At least one of the cell wall structure and each homogeneous alignment surface cooperates with the liquid crystal material so as to form focal conic and planar textures that are stable in the absence of a field. This homogeneous alignment surface is effective to increase by at least 10% a peak degree of circular polarization of light reflected from the planar texture as compared to the control display.
More specifically, in the case of the display that reflects light exhibiting a significant degree of circular polarization, each homogeneous alignment surface cooperates with the material so as to be effective in increasing brightness by at least 5% at a wavelength of peak reflection of the planar texture as compared to the control display. More specifically, brightness may be increased by at least 15% and, more preferably, by at least 30%. This homogeneous alignment surface may comprise a rubbed alignment layer material disposed adjacent the cell wall structure. The display may include a cell with one side rubbed or both sides rubbed. The display may reflect a particular wavelength of electromagnetic radiation and is suitable for grey scale, as described above.
The display with the circular polarized light feature may include a circular polarizer adjacent the cell wall structure as in the case when both sides of the cell are rubbed. The homogeneous alignment surfaces cooperate with the material effective to enable use of a driving voltage that is not substantially greater than a driving voltage of the control display. This homogeneous alignment surface is characterized by a pretilt angle of greater than about 100 as in the case of a display having opposing homogeneous alignment surfaces in one region.
Another embodiment of the present invention is directed to a stacked liquid crystal display device comprising first chiral nematic liquid crystal material and second chiral nematic liquid crystal material. Between opposing substrates are formed a first region comprising the first material and a second region comprising the second material. The first region is stacked relative to the second region. At least one homogeneous alignment surface is disposed in at least one of the first region and the second region adjacent one of the substrates so as to homogeneously align the liquid crystal director adjacent thereto. At least one of the substrates and each homogeneous alignment surface cooperates with the first material to form in the first region focal conic and planar textures that are stable in the absence of a field, and at least one of the substrates and each homogeneous alignment surface cooperates with the second material to form in the second region stable focal conic and planar textures. One of the substrates and a first homogeneous alignment surface cooperates with the material in the second region so as to be effective in preventing reflection by the focal conic texture in that region from exceeding 10% at a wavelength of peak reflection of the planar texture. A device applies an electric field to transform the first material and the second material to at least one of the focal conic and planar textures.
Reference to properties of a region or cell of the stacked display require examining performance of that cell individually, rather than the entire stacked cell display as a whole. Thus, for example, when referring to a limited focal conic reflectance of a top cell of a stacked cell display, that cell should be examined apart from its behavior in the entire stacked display, such as by placing a black back layer on the individual top cell and then examining the focal conic reflectance of the top cell. It will be appreciated, however, that the improved properties of the present invention may also be observed in terms of the performance of the entire stacked cell display where indicated, such as increased brightness of the entire stacked cell display.
In particular, in this stacked display embodiment a substrate that opposes the first alignment surface may comprise a second homogeneous alignment surface. The second region with the first and second homogeneous alignment surfaces may be disposed downstream of the first region relative to a direction of incident light. A third homogeneous alignment surface may be disposed adjacent one of the substrates in the first region. One of the substrates that opposes the third homogeneous alignment surface in the first region has an inhomogeneous alignment surface. The display enables use of a driving voltage that is not substantially greater than a driving voltage for a corresponding cell in the control display.
In another aspect of the stacked display, one of the substrates that opposes the first homogeneous alignment surface in the second region has an inhomogeneous alignment surface. The first region may include only one homogeneous alignment surface with an opposing substrate with an inhomogeneous alignment surface. In all embodiments herein, each homogeneous alignment surface may comprise a rubbed alignment layer material, such as a rubbed polyimide alignment layer material. The pretilt angle of the homogeneous alignment surface in such a cell may be greater than about 10xc2x0.
The stacked display for enhanced brightness may include a first material that has a chirality of an opposite twist sense than a chirality of the second material. At least one of the first and second liquid crystal materials may be selected from the group consisting of various chiral nematic liquid crystal materials each having a pitch length effective to reflect a selected wavelength of electromagnetic radiation such as at least one of visible and infrared radiation. The device for applying an electric field can cause the first and second liquid crystal material to assume stable grey scale states.
Another embodiment of a stacked display for enhanced brightness consists of a stacked display assembly in which the materials in both cells of the display have the same helical twist sense. Both materials may reflect at the same wavelength. In this case, enhanced brightness is achieved by sandwiching a half wave plate between the two cells. The purpose of the half wave plate is to change the handedness of the circularly polarized light.
Another embodiment is a double stacked system where a circular polarizer is sandwiched between the two cells. The use of homogeneously aligned surfaces may be similarly applied to triple or multiple stacked systems to increase the brightness or degree of circular polarization, and/or decrease focal conic reflectance, of full color or multicolor/infrared combinations. At least one of the inventive homogeneous alignment surfaces may be applied in one, two or more cells of double, triple and multiple cell stacked displays. Likewise, a circular polarizer may be inserted in the stack, as would be apparent to those skilled in the art in view of this disclosure.
In the stacked display, the first homogeneous alignment surface may cooperate with the second material so as to be effective in increasing brightness by at least 5% and, in particular, by at least 15% or 30%, at a wavelength of peak reflection of the planar texture in the second region, as well as increase by at least 10% a peak degree of circular polarization of light reflected from the planar texture in the second region. The above increases in brightness and degree of polarization may be observed in any of the stacked cells which employs at least one inventive homogeneous alignment surface.
Another embodiment of the present invention is directed to a liquid crystal display including a cell in which both sides are treated, comprising the chiral nematic liquid crystal material, substrates between which the liquid crystal material is disposed and the device for applying an electric field discussed above. Homogeneous alignment surfaces are adapted to align the liquid crystal director adjacent both of the substrates. The homogeneous alignment surfaces may be characterized by a pretilt angle of greater than about 10xc2x0 and cooperate with the liquid crystal material to form focal conic and planar textures that are stable in the absence of a field.
More specifically, this display may benefit from the enhanced brightness increase of at least 5% and, in particular, at least 15% or 30%, at a wavelength of peak reflection of the planar texture. The homogeneous alignment surfaces are preferably formed of a rubbed alignment layer material. This display may benefit from the use of liquid crystal materials that can reflect selected wavelengths of electromagnetic radiation and is suitable for grey scale. The display may include a circular polarizer adjacent one of the substrates and use a driving voltage not greater than what is employed in the control display.
The present invention offers numerous advantages such as an ability to enable displays to be tailored so as to produce a variety of properties. Displays may be made so as to have improved brightness (e.g., rubbing one side) and extremely high brightness such as when both sides of the cell are rubbed. Displays may be made so as to have very low brightness of the focal conic texture not greater than 10%. This low focal conic reflectance, such as in the case of cells that have both sides rubbed, is believed to be heretofore unattainable in cholesteric liquid crystal displays. The inventive displays also possess the unique attribute of reflected light which is to a significant degree, circularly polarized, such as cells in which both sides are rubbed. This advantageously permits the use of polarizers, which typically would not be used with cholesteric liquid crystal displays. Rather than decreasing brightness, as would be the case if a polarizer were used in a conventional cholesteric display, since the reflected light is to a significant degree circularly polarized, the polarizer permits the reflected light to pass through, and prevents the passage of backscattered light from the display. Displays may be fabricated to exhibit the inventive enhanced brightness, low focal conic reflectance and increased degree of circular polarization features, and any combination thereof, by using certain alignment layer materials as well as treatment methods such as rubbing pressure and number of rubs.
The stacked display feature illustrates the versatility of the present invention. Such displays may be designed, for example, to take advantage of the very high brightness and degree of circular polarization of a cell rubbed on both sides so as to exhibit a bistable large domain display, while increasing viewing angle by using a top cell in which the top side has an inhomogeneous alignment surface. Such a stacked cell display exhibits a combination of superior brightness, low focal conic reflectance and good viewing angle, which are believed to have been heretofore unattainable.
Yet another embodiment of the present invention is directed to a method of making a cholesteric liquid crystal display comprising applying a preselected alignment layer material to at least one substrate. The alignment layer material is treated under preselected treatment conditions to produce a first homogeneous alignment surface. Chiral nematic liquid crystal material is filled between the first alignment surface and a substrate that opposes the first alignment surface. The liquid crystal material has positive dielectric anisotropy. At least one of the opposing substrate and the first alignment surface cooperates with the liquid crystal material so as to form focal conic and planar textures that are stable in the absence of a field. The first homogeneous alignment surface aligns the liquid crystal director adjacent thereto, based upon the preselected alignment layer material (e.g., high or low pretilt angle) and treatment conditions (e.g., pressure and number of rubs), to cooperate with the liquid crystal material so as to be effective to increase brightness by at least 5% and, in particular, by at least 15% or 30%, at a wavelength of peak reflection of the planar texture. Electrical connections are applied to the substrates to enable means for applying an electric field to transform the liquid crystal material to at least one of the focal conic and planar textures.
More specifically, the alignment layer material and treatment conditions are selected to enable the first homogeneous alignment surface to be effective to prevent reflectance by the focal conic texture from exceeding 10% at a wavelength of peak reflection of the planar texture. The treatment conditions in all aspects of this embodiment may include rubbing the homogeneous alignment material at a particular pressure and for a particular number of times so as to attain the specified improvement in properties. For example, a preferred alignment surface has a pretilt angle of greater than 10xc2x0.
The display may include an opposing substrate that is either treated (homogeneous) or untreated (inhomogeneous). In the case of the method of making the display in which only one side is treated, the display may be oriented such that the substrate with the inhomogeneous alignment surface is either upstream or downstream of the substrate with the homogeneous alignment surface. The alignment layer material and treatment conditions may be selected so that the first alignment surface cooperates with liquid crystal material so as to be effective to increase by at least 10% a peak degree of circular polarization of light reflected from the planar texture.
A second homogeneous alignment layer material may be applied to the substrate that opposes the first alignment surface. Both alignment surfaces may cooperate with the liquid crystal material so as to be effective to increase by at least 20% and, in particular, by at least 100%, a peak degree of circular polarization of light reflected from the planar texture.
Another embodiment is directed to a method of making a stacked cholesteric liquid crystal display in which a second region containing liquid crystal material is stacked relative to the first region. The liquid crystal material of the second region has stable focal conic and planar textures. One or both of the first and second regions may include at least one of the homogeneous alignment surfaces so as to produce the increased brightness, low focal conic reflectance and/or increased degree of circular polarization according to the present invention.
In a preferred embodiment the first region includes the homogeneous alignment surface on both sides. The alignment material may be applied to one of the substrates in the second (e.g., upper) region and subjected to the predetermined treatment conditions effective to form a third homogeneous alignment surface. A substrate in the second region that opposes the third homogeneous alignment surface may have an inhomogeneous alignment surface. The first and second regions may be stacked so that the inhomogeneous alignment surface in the second region is upstream of the third homogeneous alignment surface as well as the first region, relative to a direction of incident light. It will be understood that use of such terms as xe2x80x9cupperxe2x80x9d and xe2x80x9clowerxe2x80x9d, and xe2x80x9cfirstxe2x80x9d and xe2x80x9csecond,xe2x80x9d are relative and should not be used to limit the invention, such terms also being capable of characterization as xe2x80x9cfront,xe2x80x9d xe2x80x9cbackxe2x80x9d or the like.
Another embodiment relates to the ability of the present invention to enable an inventive display to be fabricated with reduced cell spacing with a driver that can operate at a lower voltage or faster than in a comparative display having a cell spacing that is at least 10% greater than the reduced cell spacing, and yet have brightness that is at least as high as in the comparative display. In general, cholesteric displays require relatively high drive voltage. The display cost may be significantly reduced by reducing the drive voltage. As described in this disclosure, the present invention enables displays to be produced with enhanced brightness that in some cases may greatly exceed the brightness of typical displays. One conventional method of reducing the drive voltage is to reduce the cell spacing. However, this generally will also reduce the peak brightness. Due to the enhanced brightness feature of the present invention, inventive displays may obtain at least a conventional degree of brightness with reduced cell spacing effective to enable a lower drive voltage or faster drive to be used.
A display with reduced cell spacing according to this embodiment includes chiral nematic liquid crystal material having positive dielectric anisotropy. Cell wall structure is spaced apart at a reduced cell wall spacing contains the liquid crystal material. At least one alignment surface is effective to substantially homogeneously align the liquid crystal director adjacent thereto. At least one of the cell wall structure and each alignment surface cooperates with the liquid crystal material so as to form focal conic and planar textures that are stable in the absence of a field. Each alignment surface is effective to increase brightness at a wavelength of peak reflection of the planar texture so as to be at least as high as a brightness of the identical comparative liquid crystal display device but with inhomogeneous alignment surfaces and a comparative cell wall spacing that is at least 10% greater. A device applies a drive voltage effective to transform the liquid crystal material to at least one of the focal conic and planar textures.
The reduced cell wall spacing may be effective to enable the drive voltage to be substantially less than a drive voltage of the comparative liquid crystal display device while maintaining the same peak brightness. Reduced cell wall spacing may also be effective to increase the drive speed while maintaining the same peak brightness and drive voltage of the comparative display device.
Many additional features, advantages and a fuller understanding of the invention will be had from the accompanying drawings and the detailed description that follows.